Many mechanical power generators, such as turbines, are used in various applications to supply power in various forms for various needs. For example, aircraft engines use turbines to drive the aircraft. Typically, the shaft rotational speeds in these mechanical generators are relatively high, with turbine shafts often reaching speeds of tens of thousands of revolutions per minute. In many of these applications, unconsumed mechanical power is available and it is desirable to produce electricity from this available power. However, the high shaft rotational speeds of these mechanical generators are unacceptable for conventional alternating current power generation. In conventional electrical generators, the frequency of the electricity generated is directly related to shaft speed. If electrical power were generated from these high shaft speeds, it would be of an impractically high frequency for powering conventional electrical and electronic equipment. In addition, the shaft speeds of turbines and other mechanical power generators usually vary depending upon the load and other operating conditions. These variations in shaft speed alter the frequency and amplitude of the electrical power produced by a generator mounted on the same shaft, making the generated electricity even more unacceptable for powering conventional equipment that requires electrical power of substantially constant frequency and amplitude.
The proposed solutions to extraction, i.e. co-generation, of alternating current electrical power from high, variable speed mechanical generators have not been successful. Constant-speed mechanical gearing inserted between a variable speed shaft and an electrical generator is expensive, complex, heavy and susceptible to constant maintenance and frequent replacement. For high speed turbines, mechanical gearing is particularly impractical.
Various electromechanical apparatus for reducing the frequency of electrical power, generally termed "cycloconverters", has been proposed. Examples are described in U.S. Pat. Nos. 3,431,483 to LaFuze and 3,707,665, 3,832,625, both to Gyugyi. These cycloconverters draw electric power from the various phase windings of a polyphase generator through controlled switches connected to each phase. The phases are switched under the influence of control circuitry that frequently involves complex regulators or a constant frequency generator. The different generator phases are switched "on" and "off" at different times and the various "time slices" of power transmitted during the "on" times of the switches are combined and sometimes filtered to produce electrical power at a preselected frequency. It is notable that these cycloconverters all employ polyphase electrical generators in order to produce single phase output power. Preferably the polyphase generators driving the cycloconverters employ at least six phases to produce a smooth, lower frequency output. In the cycloconverters, whenever a generator phase is switched "on", current is drawn through the entire generator winding of that phase.
Cycloconverters intended for use in motor vehicles are described in U.S. Pat. Nos. 4,415,848 to Morishita and 4,419,618 to Gretsch. Each of these cycloconverters converts three phase power to lower frequency single phase power. The patent to Morishita describes a feedback circuit for adjusting the amplitude of the induction field in the generator to control the amplitude of the single phase output signal. A constant frequency generator is used to control the switches, meaning that changes in shaft speed will alter the output waveform in amplitude in a complex, non-controlled manner. The patent to Gretsch departs from the other cycloconverters by controllably selecting taps on transformer windings rather than switching the phases of the generator "on" and "off". The phases of the generated power are selectively connected to the various transformer taps by controllable switches to match the impedance of the generator to that of the load. The matching maximizes energy transfer and minimizes the size of generator needed for a particular installation. As in the Morishita patent, a voltage regulator is used to control the current in the field winding of the generator to control the amplitude of the lower frequency output power.